1. Field of the Invention
The present invention relates to an optical information recording medium capable of recording and reproducing information by a laser beam. In particular, the invention relates to a heat mode type optical information recording medium containing an azo dye of a specific structure and suitable for recording information by a laser beam.
2. Description of the Related Art
Information recording media (optical disks) which can record information only once with laser beams have hitherto been known. These optical disks are also called write once-type CDs (so-called CD-Rs), and have the typical structure that a recording layer comprising an organic dye, a light reflective layer comprising a metal such as gold, and further a resin protective layer are provided on a transparent disk-shaped substrate in this order in a laminated state. Information is recorded on the CD-R by irradiating the CD-R with a near infrared laser beam (usually, a laser beam having a wavelength of about 780 nm). The irradiated portion of the recording layer absorbs the laser beam to locally elevate the temperature, which produces a physical or chemical change (for example, formation of pits) to alter the optical characteristics thereof, thereby recording the information. On the other hand, reading (reproduction) of the information is usually carried out by irradiating a laser beam having the same wavelength as the laser beam for recording to detect the difference in reflectance between a deformed site (recorded area) of the recording layer by heat generation and a non-deformed site (non-recorded area).
In recent years, information recording media higher in recording density have been desired. In order to meet such a request, optical disks called write once-type digital versatile disks (so-called DVD-Rs) have been commercially available. The DVD-R of this type has the structure that two disks each comprising a transparent disk-shaped substrate on which a guide groove (pre-groove) for tracking a laser beam irradiated is formed at a track pitch (0.74 to 0.8 μm) equal to or narrower than half that of the CD-R, having provided thereon a recording layer comprising a dye, usually a light reflective layer and further a protective layer as needed in this order, or the disk and a disk-shaped protective substrate having substantially the same dimension as that of the disk are adhered to each other with an adhesive, facing the recording layer inside relative to the substrates. Recording and reproduction of information in the DVD-R are carried out by irradiation of a visible laser beam (usually, a laser beam having a wavelength ranging from 630 to 680 nm), and it is said that higher density recording than that of the CD-R is possible. As the dyes used in such optical recording materials, azo metal chelate dyes formed from azo dyes and metals are disclosed. Typical examples thereof are disclosed in JP-A-63-9577, JP-A-63-9578, JP-A-63-9579, JP-A-3-268994, JP-A-8-156408 and JP-A-9-277703 (the term “JP-A” as used herein means an “unexamined Japanese patent publication”)
Recently, high-vision television and networks such as the Internet have rapidly become widespread. Further, the start of HDTV (High Definition Television) broadcasting comes near, and a demand for large-capacity optical media for recording image information inexpensively and easily is glowing. Although the DVD-Rs secure a position as the large-capacity recording media to some degree, it is not said that they have such a sufficiently large recording capacity that a future demand can be coped with. Consequently, the development of optical disks having a larger recording capacity has been advanced in which a laser beam having a shorter wavelength than that in the DVD-Rs is used, thereby improving recording density.
In an optical information recording medium having an organic dye-containing recording layer, there is disclosed a recording and reproducing method for carrying out recording and reproduction of information by irradiating the medium with a laser beam having a wavelength of 530 nm or less from the recording layer side to the light reflective layer side. Specifically, there is proposed an information recording and reproducing method for carrying out recording and reproduction of information by irradiating an optical disk using a porphyrin compound, an azo dye, a metal azo dye, a quinophthalone dye, a trimethinecyanine dye, a dicyanovinylphenyl skeleton dye, a coumarin compound, a naphthalocyanine compound or the like as the dye of the recording layer with a blue (wavelength: 405 nm, 488 nm) or blue-green (wavelength: 515 nm) laser beam.